This invention relates to a method and an apparatus for intensifying sharpness of picture images used for a color scanner for plate making, a laser color printer, a facsimile and the like for a graphic art.
Well known is a general method for intensifying sharpness of a picture image in which a sharpness intensifying signal is first obtained by multiplying an edge signal S-U (shown in FIG. 1C), which is the difference between a sharp signal S (shown in FIG. 1A) and an unsharp signal U (shown in FIG. 1B), both obtained by the image by a coefficient K for determining the degree of the sharpness intensification of the picture image, and the obtained sharpness intensifying signal is then added to the sharp signal S thereby to obtain a sharpened signal S' (=S+K.multidot.(S-U)) as shown in FIG. 1D. However, in such a prior art method, in case the sharpness intensifying process is carried out with the coefficient K being constant, is adversely intensified a noise of low level which may be added in an image signal at a time of photoelectric conversion by means of a photo-multiplier. The intensifying of the low level noise in this process makes the noise of the image after processing also intensified, and moreover, it is impossible to selectively intensify the sharpness of the picture image in accordance with density zones.
In order to obviate such defect regarding the noise intensification as described above, is also known a method in which when a dead zone value D is set and an equation .vertline.S-U.vertline.&lt;D is satisfied, an equation S'=S is obtained without carrying out the sharpness intensifying process (U.S. Pat. No. 4,005,475). In this prior art method, however, since a device such as a photo-multiplier used for the photoelectric conversion of the picture image tends to provide a high level noise particularly on the high density side, the degree of the sharpness intensification of the image on the low density side becomes insufficient when the dead zone value D is suitable for a noise level on the high density side. In addition, although the selective intensification of the sharpness of the picture image based on the density zone can be done by changing the sharpness intensifying coefficient K due to the sharp signal, in this case, problems regarding the noise must be considered independently of the above fact.